1. Field of the Invention
The present invention relates to an apparatus and method for transferring data bursts in an optical burst switching network, and more particularly, to an apparatus and method for transferring data bursts in an optical burst switching network which are capable of efficiently transferring data bursts with a minimized delay time and a low loss rate by using a hybrid optical buffer having a feed-forward type buffer shared in each port and a feedback type buffer shared in each node, the hybrid optical buffer using fiber delay lines (FDLs) to provide class-differentiated service.
2. Description of the Related Art
An optical burst switching (OBS) technique generally involves a unidirectional reservation technique of first sending a control packet to a destination node for resource reservation and directly transferring burst data without confirming whether the resource is reserved. In this case, intermediate nodes directly transfer the burst data without performing optical-electrical conversion.
Since the data bursts are transferred without confirming whether resource is secured at all nodes on paths to the destination node, they are inevitably lost in the nodes.
To solve this problem, several loss avoidance methods have been suggested, such as a method employing a wavelength converter, a method employing an fiber delay line (FDL) based buffer, a method of discarding contending portions of data bursts, and a method using a deflection routing.
Among these methods, the method employing a fiber delay line (FDL) based buffer is the best because when contention between the data bursts occur, data bursts are delayed through fiber delay lines (FDLs) by a contention time and then transferred.
This method is similar with that used in the Internet. In the Internet, a random access memory (RAM) is used as a buffer to store data for an exact contention time and then send the data. On the other hand, the fiber delay line (FDL) based buffer can store data for a time corresponding to a fixed length of the line.
In addition, because the data bursts are transferred along an unnecessarily long path, physical loss such as signal attenuation may occur. The buffer is directly connected to a switch, and accordingly increase in a buffer size is closely related to increase in a switch size. Accordingly, determination of the buffer size is a critical issue.
Fiber delay line (FDL) based buffers may be classified into a feed-forward type buffer and a feedback type buffer. The feed-forward type buffer can be used only once. If data bursts contend with each other at an output port, a buffer having a fixed length is used. At this time, the data bursts are exited from the buffer regardless of success of burst transmission.
On the other hand, in the feedback type buffer, data bursts are circulated through the buffer until output resource is available or up to an allowed maximum number of times if the output resource is not available after one round of buffering. This provides more buffering chances and thus a higher transmission success rate, but causes excessive buffering delay and signal attenuation.
Meanwhile, in an optical burst switching network, buffers may be classified into a buffer exclusively used for each wavelength, a buffer shared in a port with a number of wavelengths multiplexed, and a buffer shared in one switching network, depending on a buffer location.
These buffers have merits and demerits according to the size of a switching fabric and a sharing degree of resource. That is, when the buffers are exclusively used for each wavelength, a transmission success rate may be high but the size of the switch increases due to there being a large number of buffers. On the other hand, when a buffer is shared in the optical burst switching network, the size of the switch structure may decrease but the transmission success rate may be low.
Meanwhile, it is critical to provide class-differentiated service in an optical burst switching network. As previously stated, once the data bursts are transferred, intermediate nodes send the data bursts through a switching operation without a process of obtaining control information through optical-electrical conversion for acquiring information about a destination node. Thus, the loss of the data bursts is a more important problem than delay time.
Accordingly, services are differentiated according to loss of data bursts. An extra offset time is conventionally introduced to provide differentiated services. In this case, a control packet is sent much earlier in order to reserve resources in advance and transfer data bursts having a high priority. In this manner, by securing output resources for data bursts having higher priority earlier than for data bursts having lower priority, class-differentiated loss rates are obtained. However, the higher the priority of a data burst, the longer it is delayed prior to transmission.
In another conventional method, when different classes are in contention for output resource, they are arbitrarily blocked at a preset rate. This enables loss rates for the classes to differ from one another in the optical burst switching network, but the loss rates are too high and resources of the optical burst switching network cannot be used efficiently.
In another conventional method, input packets of each class are laid at a pre-selected location in optical bursts. When a contention between data bursts occurs, a portion of the data burst containing a packet having lower priority is cut out and a portion of the data burst containing a packet having higher priority is transferred, thus increasing the transmission success rate of higher priority packets.
In another conventional method, a group of wavelengths that can be used by different classes is virtually divided and dynamically re-arranged according to a state of an optical burst switching network, so that when there are classes having a high priority, wavelengths are preoccupied so as not to be occupied by traffic having a low priority, thus providing class-differentiated service.
Since, in such conventional service differentiating schemes, an overall blocking rate in the optical burst switching network is divided for each class, classes having higher priority have a lower blocking rate and classes having lower priority are subject to a relatively higher blocking rate. Accordingly, there is need for a scheme capable of providing differentiated service with a low blocking rate for all classes.